1. Technical Field
The present disclosure relates to a multi-level inverter, and more particularly, to a multi-level inverter capable of performing a bypass operation without adding any separate module.
2. Description of the Related Art
A medium voltage inverter is an inverter having an input power whose root mean square (RMS) value is over 600V for a line-to-line voltage. The rating power capacity of the medium voltage inverter ranges from a few hundreds of kilowatts to a few tens of megawatts. Various medium voltage inverters are widely used in application fields including fans, pumps, and compressors. Among these medium voltage inverters, a cascaded multi-level inverter is frequently used, which generates an output phase voltage having three or more levels. The magnitude and number of levels of the output phase voltage generated from the cascaded multi-level inverter are determined according to the number of unit cells constituting the cascaded multi-level inverter.
Each phase of the cascaded multi-level inverter is formed by coupling a plurality of unit cells in series, and the three-phase output voltage of the inverter is determined by adding output voltages of unit cells constituting each phase of the inverter. If one of a plurality of unit cells included in a multi-level inverter is abnormal, the inverter can perform a bypass operation performed in a state in which an output is reduced. In the bypass operation, the abnormal unit cell is not used by short-circuiting an output terminal of the unit cell through a bypass switch, and only the other normal unit cells are used, thereby performing a normal operation of the inverter.
However, in an inverter having no bypass switch for controlling a bypass operation for each unit cell, a DC-link voltage increases due to rectification of an inverse-parallel diode of a power switch used in an abnormal unit cell. Therefore, the DC-link voltage exceeds the rating operating range of a DC-link capacitor. As a result, unit cells are damaged, and all operations of the inverter cannot be performed.